"Seau's death rocked everybody. It rocked me a lot," Bell said. "The fact that there is a chance it was football related and the fact that I was a football player, it hit home with me."

And so he quit the NFL, walking away from a free-agent contract with the Cincinnati Bengals earlier this month.

"We're getting so much money, so much glory, so much fame; we're boosting our egos so much by playing a sport that's violent and could later on risk our lives," Bell said.

He said his plan now is to act as an advocate for players, alerting them to the dangers of chronic traumatic encephalopathy, or CTE. First discovered about a decade ago in the brains of former football players, it's a degenerative disease linked to symptoms like dementia, erratic behavior, and suicide. The small cadre of doctors who study CTE have diagnosed it in dozens of now-dead NFL players.

The issue of head injuries has cast a dark shadow over football for several years. But a string of recent events, including Seau's suicide, left many wondering if the risks of football are worth the rewards.

Tom Brady, Sr., the father of Superbowl star Tom Brady, told Yahoo! Sports on Tuesday that he would be "very hesitant" to let his son play football if he had to make the decision today knowing what he does now about CTE.

"This head thing is frightening for little kids," he said. "There's the physical part of it and the mental part -- it's becoming very clear there are very serious long-term ramifications."

He was responding to a recent remark made by former NFL player Kurt Warner, who told a radio show that the idea of letting his own children play football is a "scary thing."

The NFL released a statement in February, saying that the league "has long made player safety a priority and continues to take steps to protect players and advance the science and medical understanding of the management and treatment of concussions." In 2010, the league donated a $1 million grant, no strings attached, to Boston University School of Medicine's Center for the Study of Traumatic Encephalopathy (CSTE), which researches brain disease in retired football players.

But there is increasing evidence that the way the game is played is leaving a trail of invisible injuries, even among amateur athletes who have never sustained the kind of knock-out concussions often seen on the NFL gridiron.

According to research published in this month's issue of Neurology, a football player could sustain 8,000 hits over the course of a four-year high school and a four-year college career combined.

Owen Thomas is one of the youngest players ever diagnosed with CTE. It was discovered in his brain after he committed suicide during his senior year at the University of Pennsylvania, where he was the captain of the football team. He was 21.

His mother, Katherine Brearley, had struggled to find a reason why her son, whom she described as upbeat and vivacious, would suddenly take his own life.

"I thought, there has to be smoking gun here," she said. But she had never heard of CTE when Chris Nowinski of the Sports Legacy Institute, an organization that studies the link between trauma and brain health, contacted her about donating her son's brain for research.

"I was astounded because when they asked for his brain to analyze his brain for concussion effects. I said, 'Well, he never had any concussions'. I didn't think that they would find anything. I was sure they wouldn't find anything," she said.

But they did find CTE, and Brearley said she now believes the disease was the critical factor in her son's death.

"I think if he didn't have CTE, he would not have committed suicide," she said.

"My determination as I go through each day is that some good might come out of his death," Brearley said. "That the research into brain trauma will somehow have more attention."

And indeed, doctors are now on the leading edge of discovering how young players like Owen Thomas -- with no documented history of concussions -- might have damaged their brains.

Researchers call these hits "subconcussive blows" -- moments at which the brain hits the inside of the skull, but not hard enough to sustain what a doctor would diagnose as concussion.

The question now is whether an accumulation of these lesser blows over time could cause brain damage powerful enough to lead to CTE, which has thus far mostly been documented in professional players with a history of concussions.

"How much of that is occurring? We really don't know," said Dr. Julian Bailes, Co-Director of the NorthShore Neurological Institute in Evanston, Ill., who has autopsied the brains of dozens of former NFL players.

"Several former football players who had CTE did not have a history of concussion, they didn't have it. But undoubtedly they had hundreds or thousands of head impacts through the course of their career," he said.

It's hard to know definitively who has CTE or at what point it develops, he said, because as of right now, doctors can only diagnose the disease post-mortem, by cutting open the brain.

So the question of what is happening to the brains of younger athletes looms.
"I played football for ten years. I think it's America's greatest sport," said Bailes. "But I think we're at a little bit of a crossroads now."

"We need to continue to press forward with our medical knowledge, our sports medicine knowledge and make the game safer."

The key, according to Bailes, is taking head contact out of the game, especially for young people.

"I don't think they ought to have this potential for every play to hit head-to-head, what I call gratuitous, mandatory, obligatory head-to-head contact."

That recommendation is at the heart of changes he is proposing as the medical director of the Pop Warner youth football league.

"I think we'll really look at eliminating practice contact, particularly to the head. And I think that's an area that's been sort of overlooked, that we can immediately do, that will have a big impact," he said.